PROJECT ABSTRACT Many 3-4 year olds with autism spectrum disorder (ASD) who are `preverbal' (30-50%) do not go on to develop socially related words and phrases by age 5-6 years and are reclassified as `minimally verbal', never moving past 20 functional, non-echoed words, even with exposure to evidence-based early interventions. Despite significant research of children older than 5 who are already classified as minimally verbal, little research has attempted to optimize intervention for limited language preschool children who may be more likely to develop language with appropriate, targeted, and personalized intervention approaches. Therefore, a major gap in our knowledge remains on how to place these children with ASD on a path to becoming verbal by school age, thus reducing the numbers of minimally verbal children. A key challenge in this population is that one size does not fit all: there is vast heterogeneity in presentation and response to intervention. To capitalize on this heterogeneity, it will be necessary to use an individually tailored, sequential approach to intervention whereby treatment is adapted over time based on how the child responds. The overarching goal of the proposed multi-site project is to maximize language outcomes for preschoolers at risk for maintaining `minimally verbal' status by developing a two-stage, 20-week ?adaptive intervention? (an intervention that incorporates a replicable sequence of treatment decision rules to guide clinicians on when and how to alter an intervention). This study addresses key questions in designing an adaptive intervention, including how best to intervene with children responding slowly to initial intervention, which of four pre-specified adaptive interventions is best, which moderators are important to designing a more individually tailored adaptive intervention, and better understanding the mechanism by which the adaptive interventions may exhibit their effects. Specifically, the primary aim tests the effect of intensifying stage 1 singular interventions versus changing intervention by systematically combining and enhancing treatment modules. The stage 1 interventions include a commonly recommended approach based on applied behavior analysis (discrete trial training) and an evidence-based early intervention that specifically addresses the social communication impairment in ASD, JASPER (Joint Attention Symbolic Play Engagement & Regulation). The primary outcome is spontaneous communicative utterances, and secondary outcomes include joint engagement, receptive language, and presence of word combinations. Our secondary and exploratory aims include moderator and mediator tests, and longitudinal outcome at age 5-6 years. Child participants include 140 4-year-olds with ASD and limited language (<20 functional words) across 3 sites (U Oregon, Rochester, and UCLA). This study has the potential to dramatically improve communication outcomes for children with ASD, and addresses a high priority need of the Interagency Autism Coordinating Council and NIH.